Methane, the major component of natural gas, is a potential source of hydrogen for fuelling fuel cells. However, the hydrogen in methane is chemically bound to the carbon and must be liberated by chemical processing before it can be used. The liberation of hydrogen from methane may be achieved by reforming.
One known method of reforming methane, or natural gas, is by catalytic steam reforming. In this process a mixture of steam and methane, or natural gas, is passed over a suitable catalyst at a high temperature. The catalyst may be nickel and the temperature is between 700.degree. C. and 1000.degree. C. Hydrogen is liberated according to the following overall reaction: EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2
This reaction is a highly endothermic reaction and requires an external heat supply and a steam supply. Commercial steam reformers typically comprise externally heated, catalyst filled tubes and rarely have thermal efficiencies greater than 60%.
A further known method of reforming methane, or natural gas, is by catalytic partial oxidation reforming. In this process a mixture of methane, or natural gas, and an oxygen containing gas is passed over a suitable catalyst at a high temperature. The catalyst is normally a noble metal or nickel and the high temperature is between 700.degree. C. and 1200.degree. C. Hydrogen is liberated according to the following overall reaction: EQU CH.sub.4 +O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2
This reaction is a highly exothermic reaction and once started generates sufficient heat to be self sustaining. No external heat supply or steam supply are required. The catalytic partial oxidation reforming technique is simpler than the catalytic steam reforming technique, but is not as thermally efficient as catalytic steam reforming.
An additional known method of reforming methane, or natural gas, is by autothermal reforming. In this process a mixture of methane, or natural gas, air or oxygen and steam is used to liberate hydrogen. The autothermal reformer uses a combination of catalytic steam reforming and catalytic partial oxidation reforming. The catalytic partial oxidation reforming reaction produces the heat for the catalytic steam reforming reaction. A correctly designed autothermal reformer is potentially far more efficient than a either a catalytic steam reformer or a catalytic partial oxidation reformer. Hydrogen is liberated according to the following overall reaction: EQU CH.sub.4 +yH.sub.2 O+(1-y/2)O.sub.2 .fwdarw.CO.sub.2 +(2+y)H.sub.2 O&lt;y&lt;2
Consideration of the standard enthalpies of formation shows that autothermal operation is theoretically achieved when y=1.115. At autothermal operation there is no net energy input or energy output.
In addition to the reforming reactions discussed above it is usually necessary to consider the effects of another reaction occurring, the so called "water gas shift reaction". In the water gas shift reaction the following overall reaction occurs: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
The equilibrium of this reversible reaction is temperature dependent. At high temperatures carbon monoxide and water tend to be produced, but at low temperatures carbon dioxide and hydrogen tend to be produced. Reformers produce carbon dioxide and hydrogen, and consequently some carbon dioxide and hydrogen react to produce carbon monoxide and water due to the water gas shift reaction occurring in the reforming chamber.
It is known to recover some hydrogen by passing the product gases leaving the reformer, after cooling, into a shift reactor where a suitable catalyst causes the carbon monoxide and water/steam to react to produce carbon dioxide and hydrogen. In addition to recovering otherwise lost hydrogen the shift reactor is important in fuel cell fuel processing systems because carbon monoxide acts as a severe anode catalyst poison in low temperature fuel cells, such as solid polymer electrolyte fuel cells. The shift reactor provides a convenient method of reducing the carbon monoxide content of the reformer product gases.